The invention relates to apparatus for sensing and adjusting the tension in a band saw blade, scroll saw blade, drive belt, fan belt, chain drive, and the like, and, more particularly, to a tensioning gauge for measuring the tension under which an elongated flexible member such as a saw blade is held wherein the tensioning gauge comprises, indicator means; tang means depressible from an extended position toward an activating position for activating the indicator means when the tang means is depressed to the activating position; adjustable bias means for biasing the tang means towards the extended position; and depression force calibrating means for adjusting the force required to depress the tang means to the activating position by changing a biasing force of the bias means, whereby the member bows upon being pressed against the tang means so that the tang means is not depressed to its activating position unless the member is held under at least an amount of tension proportional to the biasing force of the bias means.
Many power tools and machines have parts moving under tension which must be accurately adjusted in order for the machine or tool to operate properly. For example, in a scroll saw, a saw blade is held under tension at its two ends and a motor causes the blade to oscillate longitudinally. In a band saw, a loop saw blade passes over two wheels which can be moved apart to apply tension to the blade which moves in a constant directions as the wheels revolve. The amount of tension with which the scroll saw and band saw blades are held is critical to the proper operation of the saw, as is well known. Specifically, if a blade moves laterally too much, the operator will not be able to cut a straight line or follow a pattern. If the blade is too tight, the saw will not be able to cut tight curves or radii. This is because in cutting tight curves, the blade must cut using the sides of its teeth. The blade must flex or twist to some degree in order to allow for this necessary cutting action. The tension specified by the manufacturer for a specific blade is typically the result of extensive testing in woods ranging from very soft to extremely hard and abrasive. The exact tension setting specified by the manufacturer allows the operator to achieve all of the various cuts the blade was designed to make in the material the saw and blade are rated to cut. For this reason, a saw tension knob is provided which permits the operator to adjust the blade tension as often as necessary. Additionally, if the tension is too high a saw blade of this type will tend to break more quickly.
Two wheel or three wheel band saws have one drive wheel and one or two idler wheels. Typically, one of the idler wheels is mounted in a slide so as to be movable. When the saw tension knob is turned, the movable idler wheel moves up or down in the slide so as to increase or decrease the tension on the saw blade. The amount of movement in the slide is limited differently in band saws of different manufacturers. It is not always possible to apply sufficient tension to the blade, particularly if the blade is so long that the slide runs out of upward travel.
Some machines have a scale in the form of markings embossed in the slide to show the operator where a marker should align for various blade thicknesses. Of course, such markings are calibrated for a given blade length, which may vary significantly, particularly if the operator employs a blade sold by a different manufacturer. Also, in some instances (particularly in commercial applications where the proper tools are available) a user may buy band saw blade material in large lengths and then cut it to smaller lengths and braze the ends together to form a loop band for cutting use. The problem here is that different blades of the same type will usually vary in length. In fact, a typical band saw is designed to accomodate blade lengths which vary by as much as two inches. Therefore, replacing a worn blade with a new blade may cause the embossed scale to give a faulty reading of blade tension. Because this can cause the operator to adjust the blade tension to a value other than that specified by the manufacturer, the faulty reading decreases the performance of the saw.
Scroll saws differ from band saws in that they use standard length blades generally five or six inches long. The blades are fastened at the ends of two parallel arms or at the open end of a "C" frame. Blades used in these machines are extremely thin and very narrow. They can "bow", or move a considerable distance in their center, when cutting wood or metal. Since these machines are designed to make intricate cuts and curves, bowing causes inaccuracies in cutting the correct pattern. Accordingly, setting the blade tension accurately is critical in scroll saws as well as band saws.
Manufacturers of band saws and scroll saws generally advise users to tension the blade by increasing tension until a particular musical note can be produced by plucking the blade. Aside from the fact that few people have perfect pitch or the use of a tuning instrument to check the tone produced by the blade, different metals produce different sounds (or overtones) when held at the same tension. Thus, the typical operator has no reliable way of verifying that the blade tension equals the manufacturer's specification.
Another problem is that the operator must check the blade tension quite often, so that he cannot use awkward or time-consuming methods for checking blade tension. This is because band saws and scroll saws use a spring in the tensioning device. This spring wears over time, causing the tension to fluctuate uncontrollably. Moreover, the saw blades themselves tend to stretch over time, further complicating the fluctuation in blade tension. Thus, the operator must frequently check the saw blade tension in order to maintain the optimum saw and blade performance. The problem is that, in the prior art as it exists today, there is no reliable and efficient way of checking saw blade tension with any meaningful accuracy. Therefore, the typical operator must work with a band or scroll saw which is not operating at its optimum performance level. Similar problems exist in other tensioned devices including, but not limited to, fan belts, chain drives, and the like.
Accordingly, it is an object of the invention to provide a practical way for an operator to ensure that his band saw or scroll saw, or the like, is operating at peak performance by accurately and quickly measuring the blade tension.
It is a further object of the invention to provide a device for measuring the tension in a saw blade, or the like, without requiring the operator to employ sophisticated or acoustic instrumentation.
It is yet a further object of the invention to provide a device for quickly determining the tension in a movable member in a band saw, or the like, which is simple and easy for anyone to use.
It is an even further object of the invention to provide a device for quickly measuring the tension in a movable member in a band saw, or the like, which is very accurate.
It is a yet further object of the invention to provide a device for quickly measuring the tension in a movable member or blade in a band saw, or the like, by measuring the force required to laterally displace the member of blade.
Other objects and benefits of the invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.